It is known to determine the distribution of axial strain or temperature along the length of a fiber optic sensor according to the technique described by S. K. Yao et al. in Volume 21 Applied Optics (1982) pages 3059-3060. According to this technique, very small deformations at the interface between an optical core and its cladding will cause light measurably to couple from core to cladding modes. This permits measurements by timing-domain reflectometry or a series of cladding taps to determine transmission loss and the distribution of applied perturbations.